Many light modulators used in projection and direct viewing systems operate on the basis of polarization. Such light modulators may include reflective displays such as LCoS (Liquid Crystal on Silicon), super twisted nematic (STN), and ferroelectric (FLC) as well as transmissive displays, such as thin film transistor (TFT), polysilicon (P-si), and Silicon-on-Insulator (SOI). These displays can produce a high-resolution image by changing the polarization state upon reflection or transmission of incident light. In a LCoS display, for example, in the dark state, a pixel reflects all light with substantially no change in polarization. In the bright state, the pixel rotates the polarization state of reflected incident light to the corresponding orthogonal state. A light modulator may also include a polarization-analyzing device designed to propagate the light from the bright state as a display image to be viewed by the human eye or projected onto a viewing screen.
Because light typically contains multiple polarization states, a polarization conversion assembly is often used to convert or rotate the light from a light source to substantially one polarization state. Having light of one polarization state may allow for more efficient processing through the light modulator.
A multi-PBS can be used as a polarization conversion assembly. The multi-PBS has a two-dimensional array of small polarizing beam splitters and associated lenses. The beam splitters and lenses are precisely aligned so that the output of the multi-PBS is substantially collimated and has a single polarization state. The multi-PBS converts almost all of the input light to the same polarization state. However, it is expensive to produce due to the complex structures required and the precision required to align each of the PBS structures with each lens. The multi-PBS therefore increases the cost of the projection system.
Another disadvantage of the multi-PBS system comes from a difference in the transmissivity over different incident angles in a typical light modulator between orthogonal axes. Conventional polarization conversion assemblies, including the multi-PBS, transmit the polarized light with a substantially uniform intensity distribution. More particularly, the polarized light from a conventional polarization conversion assembly is point symmetrical around the center. The light modulator, on the other hand, may have an angular transmission distribution that is not point symmetrical around the center, resulting in a significant portion of the light from the polarization conversion assembly illuminating the light modulator at incident angles with a lower probability of transmission.